The invention relates to a magnetic field generating assembly and to apparatus for performing a nuclear magnetic resonance (NMR) experiment using such an assembly.
In the field of nuclear magnetic resonance, it is necessary to generate within a working volume a substantially homogenous magnetic field to enable the NMR characteristics of molecules within the working volume to be examined. Conventionally, this has been achieved by making use of a coil assembly at the centre of which an approximately spherical region having a homogeneous magnetic field is generated. Such an assembly is described in U.S. Pat. No. 4,656,423. This has proved satisfactory in the case of NMR imaging of the human body and also NMR spectroscopy. However, there is a need to be able to perform NMR experiments on relatively large bodies, for example polymer composites such as aircraft parts and the like and the cost of manufacturing a magnet of a conventional form into which the body can be placed is so large as to be prohibitive.
Samoilenko et al JETP Lett. Vol. 47, No 7, 10 Apr. 1988, describe how large field gradients within conventional high resolution NMR magnets can provide sufficient field gradient to facilitate a selected region of the order of 0.1 mm with the application of an rf pulse only. However, again the working region is confined within the magnet.
Recently, attempts have been made to change the location of the homogenous, working volume and in one such attempt described in U.S. Pat. No. 4,701,736 a magnet is described in which the spherical homogeneous region is projected outside the volume of the magnet. A number of nested, counter running coils are used. This suffers from the disadvantages firstly that more power (in a resistive system) or more superconducting material (in a superconducting system) is required to produce a given field strength in the working volume; and secondly the working volume is spherical which is not particularly suitable for certain applications.
A second assembly for generating a substantially homogeneous region projected from the assembly is described in EP-A-0186998 which comprises a number of coil sets with their axes substantially parallel and spaced apart and this requires an undesirable amount of superconductor (in the case of superconducting magnets) or power (in the case of resistive magnets) to achieve a given field strength.